Abstract/Summary

Projections of the synergistic effects of acid deposition and climate change on freshwater ecosystems are inherently fraught with the uncertainty that such projections are for climatic conditions not currently experienced. For many of
the climate scenarios, the projected mean temperature in the future will be well above that observed even in extreme years during the period of observation (maximum 30 years for most ecosystems). The ecosystem responses are probably
not linear; thus, extrapolation from observations, even those spanning several decades, entails going outside the range of observations. It is acid deposition that is responsible for the widespread acidification of surface waters in sensitive areas of Europe, eastern North America and elsewhere in the world. This means that measures to reduce acidification problems can continue to be focussed on reducing emissions of S and N compounds to the
atmosphere. Although reductions in emissions of S and N compounds have led to dramatic improvements and recovery in water quality in acidified freshwater ecosystems, biological recovery has lagged and the problem will remain in many areas for decades to come. Further reductions are required if the goal is to permit recovery of all impacted ecosystems. Climate change is a confounding factor in that it can exacerbate or ameliorate the rate and degree of acidification and recovery, both with respect to chemical
as well as biological effects. The absence of recovery following reduction in acid deposition, therefore, may simply be the result of the confounding influence of
climatic variations. The time-scales of recovery from acid deposition are in many respects similar to those of chronic changes in climate, in part because both drivers act by affecting large pools of S, N, C and base cations in catchment soil. But extreme climatic events, such as droughts, cause extreme responses that set back the biological recovery process and slow down progress towards a stable ecosystem. The interactions are complicated and manifold, and thus the outcomes on ecosystems are difficult to predict and generalize. Both acid deposition and climate change are caused by emissions of gases to the atmosphere and are largely due to the same types of human activities –
burning of fossil fuels and other industrial processes. Clearly, there are substantial ‘co-benefits’ to be gained: for example, reductions in emissions of CO2 by a switch to renewable energy sources will also bring about a reduction
in S and N emissions. At the policy level, much might be gained by coordinating future emission controls, now dealt with separately under the United Nations Economic Council for Europe (UNECE) Convention on Long-Range Transboundary Air Pollution (LRTAP) and the UN Framework Convention on
Climate Change. Society will take measures to ameliorate or mitigate the effects of climate change. Some of these measures may indirectly affect the acidification of
sensitive freshwaters. For example, as illustrated by the modelling example from Finland (Fig. 7.15), more intensive use of forests for biofuel may entail release of N now stored in the soil to surface waters in the form of NO3
accompanied by acidic cations. More research is needed on the effects of adaptation and mitigation.
The mechanisms of the interactions between climate effects and acidification effects are still, however, poorly understood. Experiments, continued monitoring and analysis of long-term data series and modelling are complementary approaches that lead to new insights and knowledge on possible interactions. Research is particularly challenging in this field because the goal is to make projections for
the future under climatic conditions that for many ecosystems have never been experienced previously. It is certain that climate change will have an increasing
impact on freshwaters in the foreseeable future and there will certainly be effects not yet identified. Current assessments of total impact on freshwaters are probably
underestimated.